1. Field of the Invention
This invention relates to an air-fuel ratio control method for internal combustion engines, and more particularly to an air-fuel ratio control method for feedback-controlling the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine by the use of an exhaust gas ingredient concentration sensor.
2. Prior Art
Conventionally, as an air-fuel ratio control method for an internal combustion engine having an exhaust passage, an intake passage, an exhaust gas recirculation passage connecting between the exhaust passage and the intake passage, an exhaust gas recirculation control valve arranged across the exhaust gas recirculation passage, and an exhaust gas ingredient concentration sensor arranged in the exhaust passage, there has been proposed e.g. by Japanese Patent Publication (Kokoku) No. 63-16577, in which an amount of fuel supplied to the engine is calculated by the use of an air-fuel ratio correction coefficient set in dependence on an output from the exhaust gas ingredient concentration sensor, to thereby feedback-control the air-fuel ratio to a desired air-fuel ratio in response to operating conditions of the engine, and an average value of the air-fuel ratio correction coefficient is calculated. The average value of the air-fuel ratio correction coefficient is used as an initial value thereof when the feedback control of the air-fuel ratio is resumed, or it is used in place of the air-fuel ratio correction coefficient when the feedback control is interrupted, or the air-fuel ratio correction coefficient and the average value thereof are both used, to calculate an amount of fuel supplied to the engine.
Further, an air-fuel ratio control method has been recently proposed, in which an exhaust gas ingredient concentration sensor having an output characteristic which is approximately proportional to the concentration of an exhaust gas ingredient is used in feedback-controlling the air-fuel ratio of an air-fuel mixture supplied to the engine (hereinafter referred to as "the supply air-fuel ratio") to a desired air-fuel ratio set in response to operating conditions of the engine. For example, a method of this kind has been proposed by Japanese Provisional Patent Publication (Kokai) No. 62-203951, in which an air-fuel ratio correction coefficient is calculated based on an output from an exhaust gas ingredient concentration sensor of the above-mentioned type and a desired air-fuel ratio, and the difference between an actual value of the correction coefficient and a reference value thereof is calculated as a learned value in each of steady and transient operating conditions of the engine, the learned value being used in controlling the supply air-fuel ratio.
On the other hand, an exhaust gas recirculation control method for an internal combustion engine has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 55-123345, in which an actual value of the opening of an exhaust gas recirculation control valve arranged across an exhaust gas recirculation passage connecting between the exhaust passage and the intake passage is detected, while a desired value of the opening of the exhaust gas recirculation control valve is set in response to operating conditions of the engine detected, and there is calculated the difference between the actual value and the desired value of the opening of the exhaust gas recirculation control valve, to thereby control the exhaust gas recirculation control valve in such a manner that the difference becomes equal to zero.
However, when this exhaust gas recirculation control method is used in combination with the above described air-fuel ratio control method in an internal combustion engine, the calculation of the average value of the air-fuel ratio correction coefficient suffers from the following problems:
In carrying out the air-fuel ratio feedback control, particularly the air-fuel ratio feedback control over a wide operating condition range of the engine by the use of the above-mentioned exhaust ingredient concentration sensor having the proportional output characteristic, it is necessary to calculate an average value (i.e. learned value) of the air-fuel ratio correction coefficient even in an exhaust gas recirculation control region in order to secure a wider learning range of the operating conditions of the engine. However, a basic fuel amount to be corrected by the air-fuel ratio correction coefficient when the exhaust gas recirculation is performed is set to a value suitable for an engine operating condition in which the desired value of the opening of the recirculation control valve and the actual value of same are equal to each other. Therefore, when the actual value is largely different from the desired value, the actual amount of exhaust gas recirculated is deviated from a desired value, and hence the supply air-fuel ratio is deviated from a proper value. As a result, the air-fuel ratio correction coefficient set in response to the output from the exhaust gas ingredient concentration sensor assumes a value much deviated from a proper value, which make it impossible to obtain an accurate learned value of the air-fuel ratio correction coefficient. If such an inaccurate learned value is used, the supply air-fuel ratio cannot be controlled properly. In this connection, the actual value of the opening of the exhaust gas recirculation control valve is largely different from the desired value, e.g. when the lift operation of the valve is delayed relative to a change in the engine operating condition because the engine is in a transient operating condition, such as an accelerating condition, or when the opening of the valve, which operates in response to the intake pipe negative pressure of the engine, becomes inaccurate due to lowered atmospheric pressure at high altitudes.